Poor Rooster has a cold....advise?

My beautiful Gold Laced Banty Roo started to "cough" about a week ago. I've heard birds sneeze before but never cough. He has no other symptoms, eats, crows, clucks, poops, mates all normally. He only does it at night but it got worse over a few days so I spoke to a friend who raises chickens and she told me to get Tetroxy for his water. She said to put just enough in to make the water a little yellow. I've been very faithful about mixing it, he and his 2 hens are being treated and he seems to be getting better. The question is, the instructions say to make it up fresh every day. The probelm is, they go in the coop in the morning and then I move them to a secure hutch at night (the coop isn't done completely and I don't want to leave them out over night) I have water for them in both and they don't ever drink it all so every morning and every night I'm mixing new water so i'm going through the Tetroxy very quickly. Does it really need to be mixed fresh every day or can it go a day before being changed?